of: kunit: Extract some overlay boiler plate into macros

Make the lives of __of_overlay_apply_kunit() callers easier by
extracting some of the boiler plate involved in referencing the DT
overlays.

Cc: Brendan Higgins <brendan.higgins@linux.dev>
Cc: David Gow <davidgow@google.com>
Cc: Rae Moar <rmoar@google.com>
Cc: Peng Fan <peng.fan@nxp.com>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>
Link: https://lore.kernel.org/r/20240822002433.1163814-3-sboyd@kernel.org